Television transmitter



f3 W @W m b ng ,mf//a f7 o mm Z b .MH n y Dec. 9, 1941. R. GEBAUER TELEVISION TRANSMITTER Filed Nov. 9. 1939 Patented Dec. 9, 1941 TELEVISION TRANSMITTER Rudolf Gebauer, Berlin, Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof, Lorenzweg, Germany, a company Application November 9, 1939, Serial No. 303,508

Germany November 15, 1938 'I'he invention relates to television transmitters of the kind in which an image of the object to be televised is analysed or scanned by a cathode beam moved across it in a series of parallel paths. The object to be televised is either projected directly onto a screen which has light-sensitive elements arranged to store electric charges, or is projected on a semi-transparent photocathode to produce an optic image thereon while the photo-electrons, emanating from this cathode in accordance with the luminosity values of the image, are in their turn reproduced on the lightsensitive screen in electron-optic fashion. The cathode beam used for scanning acts to restore the light-sensitive elements of the screen to an equilibrium potential, these elements being well insulated from each other and from the so-called signal plate that forms part of the screen. Television transmitters of this kind enable the charges originating on the screen either by the light itself or by the electrons impinging upon the screen, to be partially stored during a scanning period. However, the eiciency of these devices is comparatively small since only 5 or 10% of the emission of electrons from the mosaic, produced thereon by the light or by the photoelectrons, are available for the electrostatic picture. This small utilisation is due to the working conditions of the television transmitters. In fact, as is Well known, a space charge arises between mosaic and accelerating electrode through the electron bombardment eiected by the scanning beam. Such space charge on the one hand aids in scanning and therefore is necessary with signal storing cathode ray scanning tubes. On the other hand, however, the space charge also aids in producing the electrostatic image and, hence, entails the said low efliciency, because the emission of photo-electron or the adequate emission of secondary electrons is completely compensated during a part of the scanning period. This will be understood from the following.

All the potentials finally acquired by the mosaic elements during scanning as well as afterwards are equilibrium values, that is, values with which as many electrons return to the emanating mosaic element, and from the space charge in front of the mosaic, as are leaving such element, there being in this way an outgoing stream and a reverse stream which equals the former. As is well known, the positive electrostatic picture or, more particularly, the picture potential difference V1-V1 between an illuminated element and a non-illuminated one is due to a complicated reaction eiiect between space charge and the emission of electrons. When owing to the space charge the elemental capacities of the mosaic screen have been charged to an equilibrium potential of say +3 volts with respect to the accelerating electrode or anode, a stream of electrons returns' to them and acts to discharge them to an extent that depends on the space charge. A non-illuminated element, emitting no electrons, after scanning finally acquires the potential V1 which is 'such that this element ceases from receiving electrons from the space charge. Elements, however, which are illuminated and, hence, emit a stream of photo-electrons which is proportional to their potentials, are discharged to potentials V1 which' are'more highly positive than V1 and with which, as stated, the reverse stream from the space charge will be just equal to said outgoing stream. As these equlibrium values are reached after a fraction of one scanning period, the photo-emission will not affect the potential difference V1-V1 during the remaining time. Furthermore, this emission merely serves to maintain vsuch potential but does not increase it since the mosaic element receives from the space charge the same number of electrons as are added to the space charge. In this way, a positive electrostatic image which is adequate to the luminosity values of the picture is produced on the screen. However, the photoemission is utilized here in part only. Experience has shown it to beutilized to about 5 or 10% only. This drawback, which is mainly due to the cause just referred to, is attributable also rendered available for the electrostatic image.

An improvement would be to apply to theI mosaic a suitable negative bias, the acceleration, acting on the photo and secondary electrons, being improved thereby. Such bias should be calculated to provide for a certain space charge, namely, the space charge necessary for the scanning operation and which therefore must be below the saturation voltage of the secondary electrons liberated by the scanning beam.

In order to bias the mosaic elements negasecondary emission factor. is-l 1.

tively with respect to the accelerating electrode they require to be supplied with a negative charge in any suitable manner. An obvious expedient will be to supply the electrons by the intermediary of the signal plate which to such end is given a negative potential with respect to the anode, and to replace the customary mica insula-v tion of the mosaic screen by a suitable semi-conductor. In the case of screens so provided with a semi-conductive dielectric the photo-electric charge originates on a layer possessed of a finite time constant which'is about 5 times' the scan-1 1. In this Way, it is possible for the second or charging beam to supply the mosaic elements with a predetermined quantity of negative electricity and thereby to apply to it the proper negative bias. The charging beam is somewhat out of phase with the scanning beam but is moved with the same velocity as this.

The invention is thus characterized by the use of a two-side mosaic to which the said special properties are peculiar, combined with two cathode beams one of which scans the positive electrostatic image while the other, acting on the opposite side of the screen, creates the negative bias. y

at once but during a number of scanning periodsV Therefore, disturbances can arise whenV transmitting a picture of rapidly changing contents. This also accords with the fact, found' by experiment, that a moving' object appears.y to showv less contrasts than an object which: is; at a. stand-- still,.and that an object which is first at' a standstill and` is then quickly moved out of the range of vision causes aftereimagesto be 'seen'. It seems. that theseA disturbances can. be obvia-ted-r to: a great' extent by choosing the working conditions accordingly. Although the sensitivity is diminished in this way yet screens of the semi-consductor type appear tobe more sensitive than the inconoscope. byl Zvvorykin.A

The invention described hereafter not' onlyY allows of. utilizing the photo-emission. to an ex.- tent; greater than in the case.- of the iconoscope and greater also than inthe case Vof semi-conductor screens. In. addition, avoiding the use of a.y semi-conductoris of advantage inasmuch as certain disturbances which mayv be caused by a` device of this, kind. will'.` be obviated.. In accordance with the gist of the invention the space charge inn front of the mosaic: must be permittedA to arise to such. extent only as be necessary for the scanning operation, Whereas the invention'A aims to preclude the detrimental influence by which the space charge tends to affect the electrostatic' image. For this. purpose a; suitable: negative bias is applied to themosaic.

In order to achieve this, cathode ray scanning devices as provided by the invention operate with the aid of two cathode beams: instead of only one. These beams arri-ve in oppositedirectionsand are moved in synchronism over a screen capable of storing electric `charges, one beam having to' scan the mosaici elements while the other has tobias them negatively.

When employing a scanningfprocess of the kind known from the iconoscope while providing for highy light-sensitivity of the mosaic' elements, there should be a secondary emission factor of the value S 1\, whereasl for the negative charge to be produced. by the second beam a.. secondary emission factor Sf 1 is necessary. 'Iihef invention therefore requires each mosaic element.- to be. possessed ofboth' these properties.. Under the invention: each mosaic element is..V al@ ranged to-project through the screen, thus being accessible from both sides thereof. Howeven the mosaic elements so arranged are'not on; both sides` of high photo-electric sensitivity but are. so on` that side only onwhich picture projection and scanning are` eiected. On this side and with thev proper rate of moving the beam the- Gn the.. other side, the mosaic elements are provided with a substance rhaving` a secondary emission factor tures of the structureA, B. are filled with av suit` of the.V screen which in Fig. 1v isv represented.`

schematically.

In. Fig. 1, I denotes the scanning beam, II the charging beam, While M indicates. the two-side mosaicscreen. P, Q designate'scanning devices which are well-known. per se and include so;

calledv electronguns for generating the beams' I, II. l

As shown in Fig. 3; screen M comprises a ne.- meshed metal grating A which is equivalent to the wellknown. signaly plateA of they customary single-side,4 mosaic screen. Grating A is provided vvith an insulating coating B- and the aperable. metal C, as silver, for example. Screenv M thusy comprises small capacities A B., C. Onthe Yside facing the beam I eachV metal core C is covered with highly sensitive photoelectric rha-V terial. DI of Va secondary emission factor S 1,y

Whereason the other side the cores C are coated with materialv E to which a secondary emissionfactor S 1 is peculiar.y For instance, carbenor aquadag will be suitable for this purpose. Scan- 5; ning and picture projection are effected both on 5o; electrons from such photo-cathode, and which the same sider of the screen The object to be. televised is projected either directly onto the screen, as shown in Fig. 1, or onto a semi-transparent photo-cathode. The emissio-n of' photois adequate tothe optic image, is in electronoptic fashion reproduced on the screen and is.

stored there.

The operation of the novel device will beobal vious from the variation which the potentialV of al non-illuminated element as compared with an illuminatedV element undergoesv during a scanning period in the manner represented by the curves 11 and n, Fig. 2.

moved over the non-illuminated element. This case is identical with the customary form ofthe iconoscope. Curve I shows the potential varia.- tion which a non-illuminated, element undergoes. During scanning an equilibrium potential arises atv the instant of time t1. Thereupon a recharging to a new equilibrium potentiall occurs.

Depending upon the intensity of the beam and,

For simplicity, we will fork 6u the first assume that only the scanning beam isr also the charging beam II is moved across screen M with the same velocity as beam I but with a slight phase shift with respect to it, then a nonilluminated element will undergo a potential variation of the kind represented by curve II, as will be understood from the following consideration. As each mosaic element has a secondary emission factor S 1 at the end facing the beam Il, each element is supplied with an electric charge E=(S-1)inTo after beam I has left it. in is the intensity of the beam II while To is the scanning duration. The potential of the respective element is thereby decreased from the value it has when beam II impinges upon it at the instant t2. Such potential is in this way decreased by an amount E f:- T o volts during the duration To, this being the time interval tz-tz. C here is the capacity of an element whose area is defined by the cross sectional area of the beam. As the potential decrease V can be predetermined by choosing the intensity of the beam II accordingly, the potential may be given a suitable negative value Vo with respect to the anode. In this way it is possible for the mosaic to be biased negatively. This bias causes the space charge to be less than in the case of the ordinary iconoscope, the negatively biased element thus being substantially protected against the influence of electrons from the space charge. A non-illuminated element, after beam II has left it, hence continues to be of the -potential Vn until touched by beam I at the instant t4 or tu. Beam I owing to the relation S 1 raises the potential V0. Through this charging operation the element under consideration is restored into the state where it can be acted upon by the space charge. The space charge hence causes an equilibrium potential V2 to be attained by the element. Compared with an iconoscope not biased, the intensity of beam I, however, being the same, the peak equilibrium potential V2 is at that moment of a somewhat higher voltage, since the space charge has been reduced. If now at the instant t1 the beam I leaves the element the equilibrium is undone; the electrons returning from the space charge decrease the potential until instant t2 is reached, and the described operation recurs. It is of importance that the peak equilibrium potential which arises at the instant t1 is in the nature of an equilibrium state, Whereas such is not the case with potential Vo created by beam II, because ypotential Vo is below the value to which it might be recharged by the available space charge alone. There must hence be a certain voltage range from the initial value Vo onward to more highly positive potentials in which the element is practically not affected by electrons frcm the space charge, that is to say, under the prevailing circumstances the photoemission will be effectually conveyed to the anode and the resultant charging of the elements will not be greatly decreased or compensated by electrons from the space charge.

As regards an illuminated element, emitting a stream of photo-electrons which is proportional to the degree of illumination, the potential variation during a scanning period will be of the kind represented by curve III. Due to the photo-emission the peak equilibrium potential V2 `at instant t1 is somewhat higher than with a nonilluminated element, as is the case also with the ordinary iconoscope. As soon as beam I leaves the element at instant t1, thus doing away with the state of equilibrium, the retroaction between photo-emission and space charge causes the potential to decrease until the instant tz is reached, at which the beam II begins to affect it. Beam II in its turn decreases the potential during the period of sweepingthe element. On account of the photo-emission the potential thereof will not entirely coincide with the value V0. The element, negatively biased after the beam II has left it, will then readily deliver its photo-emission to the anode. The potential of this element therefore rises and the instant t4 reaches the value Vo. Beam I at this moment touches the element and thereby acts to reduce it to an equilibrium potential, whereupon the described operation recurs. As the peak equilibrium potentials in the case of illuminated and non-illuminated elements are approximately the same, the difference Vo'-V1 is the effective picture potential difference that varies in accordance With the intensities of brightness and which during the scanning operation produces an adequate drop of potential across the coupling resistance, this drop of potential being utilized in well-known manner.

It can be seen that the sensitivity of the arrangement is not only greater than that of the iconoscope but greater also than the sensitivity of semi-conductor screens.

The sensitivity of the described arrangement may be increased by means of a secondary emission amplifier associated with the mosaic screen, the increase so obtained being about 10 times the original value. To such end the optic image of the object to be televised is not produced on the screen but is produced on a semi-transparent photo-cathode. The photo-emission from this cathode, and which is adequate in its cross sectional intensities to the optic image, is in electron-optical fashion reproduced on the screen. The increase in eiiiciency is on the one hand due to the fact that the semi-transparent layer is about 3 times more highly sensitive to photoelectrons than is the mosaic, While on the other hand such increase is attributable to the amplification to which the photo-electrons impinging upon the screen are subjected by the secondary electrons generated by them and which is about 5 times the original number of electrons.

The negative bias applied to the charge-storing mosaic renders it possible readily to accelerate the electronic emission produced on the screen, such being the case with both optic and electron-optic reproduction, and to store the electrostatic charges so generated at the respective spots of the screen. The invention thus involves a considerable advance as regards the eiliciency of charge-storing television transmitters.

What is claimed is:

1. A cathode ray scanning device comprising a screen comprising a plurality of mutually insulated conductors mounted adjacent each other, the ends of said conductors forming two opposite surfaces, a coating of photo-sensitive material on the ends of said conductors formingr one of said surfaces, said material having a secondary emission factor greater than unity, and a coating of material having a secondary emission factor less than unity on the ends of said .conductors forming the other of said surfaces, means for generating a first electron beam, means for directing said beam on to said one of said surfaces, means for deilecting said beam to scan said surface at a predetermined rate, means for generating a second electron beam, means for phase with respect to said rst beam, and 5 means for projecting` a light image on saidsurface scanned by said first beam.

2; A device accordingfto claim 1,. wherein said screen is composed of a. metal grating, insulating materia-L with which` this grating is coated 1g in a manner to present apertures lined with insulating materiaL, metalcores held in placehy these apertures and reaching from one sideof e the screen to, the. other, photo-electrically sensitive material covering those ends of said cores.

which are located on one side. of the screen, andmaterial having asecondary emission factor l covering the other ends thereof.

RUDOLF GEBAUER. 

